Device for controlling the frequency of a laser beam

ABSTRACT

A system for stabilizing the frequency of a laser having the resonator reflectors mounted on the ends of a gas discharge tube by controlling the temperature of the tube in response to an optical output signal from the laser.

D United States Patent {151 3,662,279 Sandstrom et al. [4 1 May 9, 1972[54] DEVICE FOR CONTROLLING THE FREQUENCY OF A LASER BEAM R f ren Cited[72] Inventors: Unto Sandstrom, Stureparken 4, 1 14 26 UNITED STATESPATENTS Stockholm; Nils Abramson Bergtorp- Svagen Taby U", sj u NicolaiSiusgatan 7 i 12 30 Stockholm; Henlick 3,431,514 3/969 Oehman et al...33 1/945 Mum! Tradgardsgatan 32 172 3 3,517,330 6/1970 Doyle 8! al...33 1/945 Sundbyberg; Berti] Coming, ploggatan 30 3,530,402 9/1920Doyle et al ..33|/94.5 724 66 Vasteras, all of Sweden 1 PrimaryE.\-aminer-William L. Sikes Flled: 1970 Assistant Examiner-Edward 8.Bauer [2]] App]. No: 82,347 Attorney-Pierce, Scheffler & Parker [57]ABSTRACT [30] Foreign Application Priority Data A system for stabilizingthe frequency of a laser having the Oct. 31, Sweden .i resonatorreflectors mounted on [he ends of a gas di charge tube by controllingthe temperature of the tube in response to [52] U.S. Cl ..33l/94.5 anomica] output signal f the laser [5 1] Int. Cl ...H0ls 3/09 [58] Fieldof Search".. ..33 1/945 14 Claims, 4 Drawing Figures i r l P'ATENTEDMAY9 I972 SHEET 1 [IF 2 FIGS! FIG.3

lllllLllll PATENTEDMAY 9 m2 SHEET 2 [1F 2 DEVICE FOR CONTROLLING THEFREQUENCY OF A LASER BEAM The present invention relates to a device forcontrolling the frequency of the radiation emitted from a laser, thelaser being of the type, at which each of the laser reflectors isrigidly connected with each its end part of the laser.

One of the objects of the invention is to facilitate production of arelatively cheap controllable laser, for which the coherence length ofthe radiation emitted from the laser is of a magnitude of several metersor tens of meters. A laser of this type is i.a. useful for measuringdistances according to the interferometer method, but the invention isnot bound to this special range of application but can be used alsowithin other fields, where laser light with a great coherence length isrequired.

That the radiation emitted from the laser has a long coherence lengthmeans that this radiation consists of essentially only one frequency. Agas laser can, however, oscillate at a number of different frequencies,provided the double length of the laser tube is a whole multiple of thecorresponding wave lengths, and that the wave lengths are within thespectral line of the gas, the width of this line being decided by theDoppler effect owing to the thermal velocity of the light emitting gasatoms. In order to achieve a laser under these conditions, which emitsonly one frequency, the laser can be made so short that only onefrequency is embraced within the special line of the gas. Onedisadvantage at this embodiment is the difficulty to obtain anamplification in this short tube exceeding the losses, and moreoverthere are difficulties in maintaining the distance between thereflectors so constant that one of the resonance frequencies of thelaser is always within the spectral line of the gas. For these reasonsthe price of such a laser will be high at the same time as the effect ofthe output radiation will be very low.

By using the present invention these disadvantages are avoided. Thecondition has been used that at a laser of the type described above, atwhich each laser reflector is rigidly connected with each one of the endparts of the laser, each longitudinal oscillation mode maintained in thelaser has a linear polarization, which deviates from the linearpolarization of adjacent longitudinal oscillation modes. If no specialmeasures are taken these linear polarizations for reciprocallyoscillation modes are perpendicular to each other, and it is supposedthat this is the case also at the laser included in the embodiment ofthe invention described below.

The explanation of the fact that at a laser of the type described above(without the Brewster window) the different longitudinal modes arelinearly polarized; every other in each of two perpendicular planes, maybe that some inconsiderable assymmetry of the laser gives preference toa certain polarization direction. The wave length, which is nearest tothe middle of the spectral line and is thus the strongest one, can thenbe assumed to take away all the energy from the atoms contributing inthis polarization direction. For adjacent wave lengths only the energywill remain that is present in the remaining atoms, and therefore thesewill be polarized in a perpendicular y direction.

The desired control of the frequency of the radiation emitted from thelaser has according to the invention been achieved by the laser havingsuch a restricted distance between the reflectors that substantiallyonly two to five oscillation modes are amplified by the laser. Inaddition to this a detector and means combined with this, which independence of a part of the radiation emitted from the laser controlmeans for adjusting the optical distance between the laser reflectors,are adapted to selectively sense the radiation in one of theseoscillation modes and to adjust said optical distance so that thefrequency of this oscillation mode is stabilized relative to the maximumpoint of the spectral line of the gas active in the laser.

According to especially advantageous embodiment of the invention saidmeans comprise means for actuating the temperature of the gas dischargetube of the laser. This actuation may consist of cooling or heating,completely or in part, of this tube.

The invention will be described below with reference to the encloseddrawings, wherein FIG. I shows the spectral line of the gas relative tothe frequencies of a number of different longitudinaloscillation modes,

. FIG. 2 and 4 show two embodiments of the invention and FIG. 3 shows anumber of curves for explaining the function of the embodimentsaccording to FIG. 2 and 4.

In FIG. 1 l0 designates the spectral line of the gas active in thelaser, thus, the amplitude of the emitted radiation in dependence of thefrequency f. Asis well-known, the spectral line takes this form onaccount of the Doppler effect, thus the influence of the thermalvelocity of the light emitting gas atoms either being in phase or incounterphase relative to the light wave passing for the moment throughthe laser. A condition for the function of the laser is that standinglight waves can be formed in the laser tube, which occurs when amultiple of the wave length of the light is equal to the double distancebetween the laser reflectors. As a consequence of this condition thedistance in frequency between the various standing waves, which can bemaintained in the laser tube, will be so small in a long laser of themagnitude 30 cm or more that a relatively great number of longitudinaloscillation modes can be embraced within the spectral line of the laser.If, on the other hand, the laser is made shorter, i.e. of a magnitude of10 cm, it can be achieved that substantially only the frequency of oneoscillation mode is contained within this spectral line. As previouslymentioned, in the latter case difficulties may arise in making the laserfunction, and moreover the effect of the emitted radiation will berelatively low.

At the laser of the invention the length of the laser tube is chosen sothat the frequencies of substantially only three to five oscillationmodes are contained within the spectral line. According to FIG. 1 threeoscillationmodes I1, 12 and 13 have such frequencies that they arewithin the spectral line. As is apparent from the above the longitudinaloscillation mode 12 has a certain linear polarization and theoscillation modes 1 l and 13 a polarization that is perpendicular tothis.

In order to stabilize the frequency of the oscillation mode 12 so thatit will be within the desired part of the spectral line 10 the device ofFIG. 2 can be used. This stabilization may aim at holding theoscillation mode 12 symmetrical relative to the spectral line 10 so thatthe emitted radiation with a frequency corresponding to this oscillationmode has maximum amplitude, but it is also possible, as appears from thefollowing, to stabilize an oscillation mode so that it lies beside themaximum point of the spectral line, and then preferably within a range,where the spectral line has its maximum inclination.

In FIG. 2 14 designates a casing enclosing a laser of a type previouslydescribed. This laser, which in the drawing is indicated with dashedlines 15, thus comprises an elongated tube with its ends parallell toeach other. The two laser reflectors are combined with these ends andpreferably within the discharge tube. The right laser reflector 16 isassumed to have a greater transparency for the radiation produced in thelaser than the lefi reflector I7, and therefore the radiation from thereflector 16 can be sent to a device (not shown in the drawing) forutilizing the laser light, for instance a distance measuringinterferometer. In respect of the fact that also at a laser tube of thistype the output light is polarized in two directions perpendicular toeach other, a polarized filter 19 is inserted in the output laser beam18, which filter isassumed to be rotatable,

so that its polarization direction can be brought to agree with thepolarization direction of the longitudinal oscillation mode, whoseposition relative to the spectral line 10 is stabilized in a mannerdescribed below.

The left laser reflector 17 shows a less degree of transparency, and thelight beam from this reflector is led to a detector 20 containing aphoto cell (not shown in the drawing) with a connected alternatingvoltage amplifier. Moreover a polarizing filter 21 is inserted betweenthe laser 15 and the detector 20, which is rotated by means of anelectric motor 22 receiving current via connectors 23.

By the influence of the rotating polarizing filter 21 the amplitude ofthe light having a certain linear polarization, which meets the photocell in the detector 20, and the corresponding output signal from theamplifier will obtain a form as indicated by the curve 24 in FIG. 3,which show the amplitude in dependence of the rotation; angle of thepolarized filter 21, v representing 360 rotation of the filter. Theamplitude will thus reach a maximum value twice per revolution of thefilter 21.

At the same time the photocell in the detector is hit by radiation witha perpendicular polarization direction, and at the output of theamplifier this will cause a signal according to the curve 25. If it isnow assumed that the curve 24 represents the radiation of thelongitudinal oscillation mode, whose position is to be stabilizedrelative to the spectral line 10, the signal according to the curve 25must in some way or other be suppressed. This is achieved by adding thesignal from the amplifier in the detector to one of the inputs of asynchronous amplifier 26 with two inputs, to the second input of which acontrol signal from the motor 22 is led. The object of this controlsignal is to make the synchronous amplifier conductive only for theperiods, when the output voltage from the amplifier in the detectoraccording to the curve 24 has its maximum amplitude. This control signalcan in a way known per se be produced if the motor 22 contains astationary winding preferably fed with DC. current, which cooperateswith a rotatable winding connected to the motor shaft, in which avoltage is produced by induction, whose frequency agrees with therotation frequency of the filter 21. In this-way the voltage 27 shown inFIG. 3 is obtained, whose phase position relative to the voltages 24 and25 can be varied in a way known per se, for instance by rotating theadjustable stationary winding in the motor 22. The voltage 27 can thusbe dephased so that its maximum, as indicated in FIG. 3, coincides withthe maximum of the voltage of the curve 24. Thus an output voltage isreceived from the synchronous amplifier 26, which is sent to a relaymeans 28. The purpose of this is to control the current supply to anelectrical motor 29, which in the embodiment of the invention shown inFIG. 2 drives a ventilator 30, by means of which air is blown into thecasing 14 of the laser 15. The device described functions in thefollowing way for stabilizing a certain longitudinal oscillation moderelative to the spectral line of the gas active in the laser. When thelaser is connected in its cold state, there is an optical distancebetween the two reflectors corresponding to certain longitudinaloscillation modes, which according to the above can be embraced withinthe spectral line of the gas. The number of these oscillation modes canbe assumed to be three. During the operation of the laser, however, aheating of the discharge tube will take place, which causes an increaseof the optical distance between the reflectors meaning in its turn thatthe modes 1 1, I2 and 13 are displaced relative to the spectral line 10.The original oscillation modes will thus gradually fall out: side thespectral line and be replaced with other oscillation modes. Thisdisplacement takes however place with increasingly lower velocityaccording as the laser approaches its continuity state. In a certainmoment the control circuit comprising the detector 20, the synchronousamplifier 26 and means connected to the amplifier is connected in. Thiscircuit can be assumed to function so that when an output signal isemitted from the synchronous amplifier 26 to the relay means 28, thecurrent to the motor 29 is disconnected so that the ventilator 30 stops.Heating of the laser tube 15 is resumed, which means that the laser tubeis extended so that the selected oscillation mode 12 leaves the maximumpoint of the spectral line 10. Therefore also the amplitude of thevoltage according to the curve 24 will be reduced, and when the voltagehas reached a determined minimum value the output signal from thesynchronous amplifier 26 is interrupted, which causes the motor 29 tostart again so that the laser tube 15 is again cooled by air from theventilator 30. This cooling continues until the oscillation mode 12 hasbeen brought back to the maximum point of the spectral line 10, afterwhich the operation continues so that this oscillation mode is thusstabilized relative to the spectral line 10.

In the foregoing it has been assumed that the air from the ventilator 30causes cooling of the laser tube 15. It is however in principle alsopossible to have the adjusting means control 7 additional heating of thelaser tube, for instance by a ventilator 30 blowing hot air towards thelaser tube. In order that the I device should then function the lasertube is brought to reach its continuity state, after which theadditional heating is made active and the laser in a manner previouslydescribed in principle is locked to an oscillation mode, which will thenappear in the laser tube.

It is also possible to add extra heating to the laser tube by providingit with external metallization, to which electric current is supplied byconnectors, the size or duration of the cur rent being controlled by therelay means 28 in substantially the same way as the heating air flow.This embodiment has among other things the advantage that the inertiaofthe system will be small.

At the embodiment shown in FIG. 2 the separation between the twopolarization directions has been achieved with electrical means, thus inthe synchronous amplifier 26, which is controlled by a signal derivedfrom the motor 22 driving the polarizing filter 21. In FIG. 4 it isshown how this separation can be achieved optically. Thus a Nicol prism31 is inserted in the light beam coming out through the left laserwindow, which prism divides the beam from the laser tube 15 into twolight beams 32 and 33, which have perpendicular polarization directionrelative to each other. Each ot these light beams hits a photo restitor34 and 35 respectively, which are included in a bridge, the otherbranches of which contain fixed resistors 36 and 37. The bridge is fedwith D.C. voltage from a voltage source 38, a center of tapping of whichcan be assumed to be earthed. Furthermore, the junction between thefixed resistors 36 and 37 is connected with earth. Finally as isindicated in FIG. 4 the junction between the photo resistors 34 and 35is connected with-the relay means 28, and it is realized thatwhen thebridge is balanced, .there is no voltage in this conductor between thebridge and the relay means 28 and that voltage is supplied to the relaymeans 28 with positive or negative polarity when one of the beams 32 and33 is stronger than the other beam. This change of polarity can thus beused for controlling the current to the motor 29 in substantially thesame way as described in connection with FIG. 2. I

Also other modifications are possible within the scope of the followingclaims. Thus it is possible instead of what has been shown in FIG. 2 and4 to have the detector actuated by the light coming through the laserreflector 16, part of this light from the reflector being deflected bymeans of a semitransparent reflector towards the detector 20, while therest of this light can be supplied to a device for using the light, forinstance a range finder, via the polarizing filter 19.

At the embodiments described in connection with FIG. 2 and 4 the airsupply to the laser tube 15 has been adjusted by starting and stoppingthe ventilator 30, but it is obvious that the same effect can beachieved by means of a valve arrangement controlled by the relay means28, which either interrupts or passes the air fiow to the laser tube 15.Furthermore, the relay means 28 might be designed so that it can controlthe speed of the motor 29 and consequently also continuously change theamount of air supplied to the laser tube 15 from the ventilator 30.

We claims:

1. A laser device for controlling the frequency of the radiation emittedfrom the laser, each of the oscillation modes maintained in the laserhaving a polarization direction which is perpendicular to thepolarization direction of adjacent oscillation modes, in addition towhich part of the radiation emitted from the laser is supplied to adetector, the output signal of which is fed to means for controlling thetemperature of the distance between the laser reflectors which aremounted on the ends of the laser tube, characterized in that the laserhas such a limited distance between the reflectors that substantiallyonly two to five oscillation modes are amplified by the laser, and thatthe detector and sensing means combined with it selectively sense theradiation in one of the polarization directions, in addition to which asignal produced at this selective sensing means is supplied to thetemperature controlling means, which adjust the optical distance betweenthe laser reflectors so that the frequency of oscillation modescorresponding to the sensed polarization direction is controlledrelative to the maximum point of the spectral line of the laser widenedby the Doppler effect.

2. Device according to claim 1, characterized in that the laser used isa gas laser.

3. Device according to claim 2, characterized in that the laser isdesigned so that each one of the laser reflectors is rigidly connectedwith each one of the end parts of the gas discharge tube of the laser.

4. Device according to claim 2, characterized in that said temperaturecontrolling means influence the temperature of the tube used in the gaslaser by controlling the temperature in at least one part of said tube.

5. Device according to claim 4, characterized in that the laser tube isprovided with a metallization, to which an electrical current heatingthe laser tube is supplied by connectors, the size of said current beingcontrolled by the output signal from the selective sensing means.

6. Device according to claim 4, characterized in that said temperaturecontrolling means produce a temperature controlling air flow, which isdirected to the laser tube and the effect of which is controllable.

7. Device according to claim 6, characterized in that the output signalfrom the selective sensing means is arranged to adjust the time duringwhich the air flow is led towards the laser tube.

8. Device according to claim 1, characterized in that in the beam pathbetween the laser and the detector, means are included, whichalternately transmit the two polarization directions for achieving aperiodical variation of the radiations reaching the detector.

9. Device according to claim 8, characterized in that said means foralternately transmitting the two polarization directions comprise arotating polarizing filter.

10. Device according to claim 9, characterized in that the filter isconnected to means for producing an electrical voltage representing theinstantaneous position of the filter relative to the polarizationdirections of the radiation from the laser towards the detector.

11. Device according to claim 10, characterized in that this voltagetogether with the output voltage from the detector is supplied to anamplifier in such manner that only part of the output voltage from thedetector and corresponding to one of the polarization directions isamplified and is supplied to the means for adjusting the opticaldistance between the reflectors.

12. Device according to claim 11, characterized in that the outputvoltage from the amplifier is supplied to a relay means controlling thecurrent supply to a motor, which drives a ventilator for sending an airflow to the laser tube.

13. Device according to claim 1, characterized in that the detectorreceives radiation from the laser tube through the least transparentreflector in the laser, a second, adjustable, polarizing filter beinglocated outside the other reflector, whose polarization direction can bebrought to agree with the polarization direction of the oscillationmode, which via the detector and means combined with it controls thedistance between the reflectors.

14. Device according to claim 1, characterized in that the radiationfrom the laser is divided into two components having perpendicularpolarization directions each one being supplied to one of two photoresistors included in a bridge connection,

the out ut voltage from the brid e being supplied to said means orcontro ling the optical tstance between the laser reflectors.

1. A laser device for controlling the frequency of the radiation emittedfrom the laser, each of the oscillation modes maintained in the laserhaving a polarization direction which is perpendicular to thepolarization direction of adjacent oscillation modes, in addition towhich part of the radiation emitted from the laser is supplied to adetector, the output signal of which is fed to means for controlling thetemperature of the laser tube which controls the adjustment of theoptical distance between the laser reflectors which are mounted on theends of the laser tube, characterized in that the laser has such alimited distance between the reflectors that substantially only two tofive oscillation modes are amplified by the laser, and that the detectorand sensing means combined with it selectively sense the radiation inone of the polarization directions, in addition to which a signalproduced at this selective sensing means is supplied to the temperaturecontrolling means, which adjust the optical distance between the laserreflectors so that the frequency of oscillation modes corresponding tothe sensed polarization direction is controlled relative to the maximumpoint of the spectral line of the laser widened by the Doppler effect.2. Device according to claim 1, characterized in that the laser used isa gas laser.
 3. Device according to claim 2, characterized in that thelaser is designed so that each one of the laser reflectors is rigidlyconnected with each one of the end parts of the gas discharge tube ofthe laser.
 4. Device according to claim 2, characterized in that saidtemperature controlling means influence the temperature of the tube usedin the gas laser by controlling the temperature in at least one part ofsaid tube.
 5. Device according to claim 4, characterized in that thelaser tube is provided with a metallization, to which an electricalcurrent heating the laser tube is supplied by connectors, the size ofsaid current being controlled by the output signal from the selectivesensing means.
 6. Device according to claim 4, characterized in thatsaid temperature controlling means produce a temperature controlling airflow, which is directed to the laser tube and the effect of which iscontrollable.
 7. Device according to claiM 6, characterized in that theoutput signal from the selective sensing means is arranged to adjust thetime during which the air flow is led towards the laser tube.
 8. Deviceaccording to claim 1, characterized in that in the beam path between thelaser and the detector, means are included, which alternately transmitthe two polarization directions for achieving a periodical variation ofthe radiations reaching the detector.
 9. Device according to claim 8,characterized in that said means for alternately transmitting the twopolarization directions comprise a rotating polarizing filter. 10.Device according to claim 9, characterized in that the filter isconnected to means for producing an electrical voltage representing theinstantaneous position of the filter relative to the polarizationdirections of the radiation from the laser towards the detector. 11.Device according to claim 10, characterized in that this voltagetogether with the output voltage from the detector is supplied to anamplifier in such manner that only part of the output voltage from thedetector and corresponding to one of the polarization directions isamplified and is supplied to the means for adjusting the opticaldistance between the reflectors.
 12. Device according to claim 11,characterized in that the output voltage from the amplifier is suppliedto a relay means controlling the current supply to a motor, which drivesa ventilator for sending an air flow to the laser tube.
 13. Deviceaccording to claim 1, characterized in that the detector receivesradiation from the laser tube through the least transparent reflector inthe laser, a second, adjustable, polarizing filter being located outsidethe other reflector, whose polarization direction can be brought toagree with the polarization direction of the oscillation mode, which viathe detector and means combined with it controls the distance betweenthe reflectors.
 14. Device according to claim 1, characterized in thatthe radiation from the laser is divided into two components havingperpendicular polarization directions each one being supplied to one oftwo photo resistors included in a bridge connection, the output voltagefrom the bridge being supplied to said means for controlling the opticaldistance between the laser reflectors.